Inputs For Yeast Calc Using Leftover Starter

[THVbrew]

I have recently started overbuilding my yeast starters in order to save some yeast for future batches. I use the yeast calculator on brewunited to do this.

I am now ready to use my mason jar with yeast from an overbuilt starter for my next batch. I plan on overbuilding it again in order to save this yeast yet again. What would be a good figure to use for the initial cell count and the viability %?

Does the following logic make sense ... Built a 1.5 liter starter with an estimated 300B cells. For the 2.5 gal. batch of beer I used half the starter and saved the other half. Is it then safe to assume I have saved 150B cells?

Thanks for anyone that can bring some clarity to this for me!

 

[LLBeanJ]

Does the following logic make sense ... Built a 1.5 liter starter with an estimated 300B cells. For the 2.5 gal. batch of beer I used half the starter and saved the other half. Is it then safe to assume I have saved 150B cells?

Yes, this is one way to do it and it's about as good as any, short of breaking out the lab equipment. And, now that you have a reasonable guestimate on cell count, viability would be determined by the date you finished the starter from which those cells were harvested. I'm not familiar with the calculator you cited, but the ones I use have a date field you populate to calculate viability.

Edit: I just checked out the Brewunited calculator and see that it HomeBrewDad's calc. And it does have a production date field for assessing viability.

 

[THVbrew]

is there any way to estimate cell count from a harvested yeast from a can that you have slowly built up? i feel like i saw somewhere that measuring the depth of yeast in your mason jar can provide some sort of count?

 

[LLBeanJ]

You can take the volume of your slurry in ml x the cell density per ml. A slurry from a starter is going to be almost pure yeast and will have a cell density of about 3 billion cells per ml. Top cropped yeast will be a little higher, maybe 3.5-4 billion cells per ml. Harvested slurry from the bottom of a fermenter is going to be around 1-2 billion cells per ml. From there you can do the math to estimate the cell count in a jar.